2.3 Conceptual modelling for the Clarence-Moreton bioregion

Executive summary

The Clarence-Moreton bioregion spans across north-east NSW and south-east Queensland, covering an area of about 24,292 km2, approximately 9,500 km2 of which is in Queensland. In NSW it contains much of the Clarence and Richmond river basins, while in south-east Queensland it covers the mid and upper parts of the Logan-Albert river basin, Bremer river basin, Lockyer Valley, and parts of the Brisbane river basin.

Conceptual models are abstractions or simplifications of reality. During development of conceptual models, the essence of how the key system components operate and interact is distilled. In bioregional assessments (BAs), conceptual models are developed to describe the causal pathways, the logical chain of events ‒ either planned or unplanned ‒ that link coal resource developments to water and water-dependent assets. This product presents information about the conceptual model of causal pathways for the Clarence-Moreton bioregion.

Methods

The conceptual model of causal pathways for the Clarence-Moreton bioregion considers two potential futures:

baseline coal resource development (baseline): a future that includes all coal mines and coal seam gas (CSG) fields that are commercially producing as of December 2012

coal resource development pathway (CRDP): a future that includes all coal mines and CSG fields that are in the baseline as well as those that are expected to begin commercial production after December 2012.

The difference in results between CRDP and baseline is the change that is primarily reported in a bioregional assessment. This change is due to the additional coal resource development (ACRD) – all coal mines and CSG fields, including expansions of baseline operations, that are expected to begin commercial production after December 2012. In the Clarence-Moreton bioregion, no new coal mines or expansions of existing mines are proposed.

Following the methods described in companion submethodology M05 (as listed in Table 1) on the development of conceptual models, this product includes:

the key system components, processes and interactions

ecosystem landscape classifications

coal resource development baseline, CRDP and the ACRD

potential hazards from coal resource development using the Impact Modes and Effect Analysis (IMEA) method

the causal pathway groups from coal resource development to hydrological change for the Clarence-Moreton bioregion. The causal pathways have been discussed with stakeholders at a workshop in June 2015.

Summary of key system components, processes and interactions

Three-dimensional geological models were developed to improve the understanding of geology and hydrogeology, and explain spatial hydrological processes in the Clarence-Moreton bioregion.

The assessment of multiple lines of evidence confirmed the role of the Lamington Volcanics as the major preferential recharge area within the Clarence-Moreton bioregion – particularly in the Richmond river basin. Recharge rates to these volcanic aquifers are at least one order of magnitude higher than recharge rates to sedimentary bedrock units such as the Walloon Coal Measures. However, a large proportion of recharge to the Lamington Volcanics discharges into the streams or the alluvium locally following short flow paths and short lag times, with only a small proportion percolating to deeper sedimentary bedrock aquifers. The assessment of the spatial distribution of median streamflow rates and hydrochemical data further confirms the significance of the Lamington Volcanics as a major hydrological feature in the Richmond river basin, where most of the surface runoff is generated. Hydrochemical data also indicate that there is aquifer connectivity between alluvial and sedimentary bedrock aquifers in some areas within the Richmond river basin. However, ideally, this would need to be confirmed independently through the use of environmental tracers.

Although multiple lines of independent evidence have resulted in a sound understanding of most of the key hydrological processes, the assessment highlighted that there continues to be conceptual uncertainties regarding the role of faults that act as potential pathways linking deeper stratigraphic units such as the Walloon Coal Measures (the CSG target), to shallow aquifers and surface water features in the Richmond river basin. Furthermore, there continues to be uncertainty on the connectivity between deep and shallow aquifers due to a lack of nested groundwater monitoring sites where groundwater levels in different aquifers are monitored simultaneously to assess vertical groundwater fluxes between different stratigraphic units.

Ecosystems

The ecosystems of the Clarence-Moreton bioregion are classified in terms of landscape classes and their dependence on water. The classifications are based on key landscape properties related to patterns in geology, geomorphology, hydrology, ecology and human-modified land use.

These landscape classes are expressed as a percentage of the geographic area associated with the Clarence-Moreton bioregion in which the potential water-related impact of coal resource development on assets is assessed. This geographic area is called the preliminary assessment extent (PAE). Most of the PAE of the Clarence-Moreton bioregion (60.3 %) is modified landscape, with by far the largest landscape class being ‘Dryland agriculture’ (57.5%). Natural vegetation landscape classes cover 37.8% of the PAE, with the ‘Woodland’ landscape class being the most prevalent of these (23.3%), then ‘Open forest’ landscape class (8.0%) and ‘Rainforest’ landscape class (5.2%).

Coal resource development

As of mid-2015, the baseline includes one existing coal mine (Jeebropilly Mine, west of Ipswich) and the CRDP includes the Jeebropilly Mine and one additional CSG development (Metgasco Limited’s West Casino Gas Project). This CSG development is located in the Richmond river basin near Casino, NSW. The focus on this area is due to the presence of highly gas-saturated coal seams that have a relatively high permeability, which are located along the western side of the Casino Trough at depths as shallow as 250 m. A recent decision by Metgasco (16 December 2015) to sell back their petroleum exploration licences (PELs) and petroleum production license application (PPLA) to the NSW Government effectively means that future development of any CSG resources in the Clarence-Moreton bioregion is highly uncertain. However, as per companion submethodology M04 for developing a coal resource development pathway, once the CRDP is determined, it is not changed for BA purposes, even in cases such as this where Metgasco have discontinued their operations in the Clarence-Moreton bioregion.

Hazard analysis

Identification of potential hazards followed the IMEA method. IMEA is used to systematically identify activities that may initiate hazards, defined as events, or chains of events that might result in an effect (a change in the quality and/or quantity of surface water or groundwater).

The hazard analysis for the Clarence-Moreton bioregion is based on the proposed CSG operations and associated water management. The assessment of geology and hydrogeology demonstrated that there is no hydraulic connection between the Richmond river basin and the Bremer river basin, where the only existing baseline coal mine (Jeebropilly Mine) is located. Consequently, potential hazards associated with coal mines are not considered in the Clarence-Moreton BA. A large number of potential hazards associated with CSG operations are identified; some of these are beyond the scope of a BA and others are adequately addressed by site-based risk management processes and regulation.

Hazards associated with coal mines and CSG operations that are considered to be in scope for BAs are grouped into four main causal pathway groups (refer to Appendix B in companion submethodology M05 for developing a conceptual model of causal pathways):

‘Subsurface depressurisation and dewatering’

‘Subsurface physical flow paths’

‘Surface water drainage’

‘Operational water management’.

These causal pathway groups are used generally across BAs; however, for the ‘Subsurface depressurisation and dewatering’ causal pathway group, no further discussion on dewatering will occur in relation to the Clarence-Moreton bioregion as dewatering applies to coal mining activities only and there are no coal mines being modelled in the baseline or CRDP for the Clarence-Moreton bioregion.

Causal pathways

The ‘Subsurface depressurisation and dewatering’ causal pathway group includes coal mine and CSG operations that intentionally dewater and depressurise subsurface hydrostratigraphic units (such as coal seams and aquifers) to permit coal resource extraction. Subsurface depressurisation associated with CSG development has the potential to directly affect the regional groundwater system at point, local or regional scales. In the Richmond river basin, there are several aquitards that can potentially prevent the wider impacts of depressurisation on overlying aquifers. However, if these aquitards were compromised by faults, the pressure change could be transmitted much faster with potential impacts extending to the uppermost aquifers.

The ‘Subsurface physical flow paths’ causal pathway group involves physical modification of the rock mass or geological architecture by creating new physical paths that water may potentially infiltrate and flow along. The extent of these changes is likely to be minor and limited to the vicinity of the compromised well (<1 km) or the location where hydraulic fracturing occurs.

‘Surface water drainage’ is the most common causal pathway group for CSG operations in the Clarence-Moreton bioregion. Subsidence, diverting site drain lines, rainwater and runoff diversion, levee bunds and creek crossings can change, or disrupt, surface water drainage. Effects on surface water direction, volume and quality can have medium-term (5 to 10 years), to long-term (10 to 100 years) cumulative effects on watercourses within and downstream of the CSG development.

The ‘Operational water management’ causal pathway group involves the modification of water management systems and may have a cumulative effect on surface water catchments and stream networks, surface water – groundwater interactions, and groundwater conditions. Effects are likely to be in the medium to long term and include watercourses in aquifer outcrop areas that are within and downstream of the CSG development.

A synthesis section summarises how all sections of this product link together, and how the conceptual model framework presented in this product will inform subsequent companion products of the BA. A qualitative assessment indicated that 21 of the 35 landscape classes identified in the Clarence-Moreton bioregion are unlikely to be impacted by any of the four causal pathway groups. The criteria adopted to undertake this assessment related to the spatial scale associated with the impact, geological and hydrogeological settings, tidal influence, the explicit location of the landscape class relative to the West Casino Gas Project, and the type of water dependency. This initial qualitative assessment will be tested in product 2.6.1 (surface water numerical modelling) and product 2.6.2 (groundwater numerical modelling) for the Clarence-Moreton bioregion where numerical modelling outcomes are reported. This means that even if the initial qualitative assessment suggests that a causal pathway does exist, the groundwater numerical model may indicate otherwise; note that the opposite scenario is also possible.

Gaps

Knowledge gaps for the conceptual model of causal pathways in the Richmond river basin are primarily related to geological and hydrogeological data gaps (e.g. the general lack of deep groundwater monitoring bores, the lack of hydraulic property data of aquifers and aquitards and the lack of nested groundwater monitoring sites), data quality issues and the complex nature of the geological and hydrogeological setting. Furthermore, the assessment of landscape classes showed that not all landscape classes may be represented in the groundwater modelling area and not all spatial data may be obtained for each landscape class (spatial data was either non-existent or non-accessible within the window of data collection for the BA analysis).

Further work

The causal pathways described in this product guide how the modelling (product 2.6.1 (surface water numerical modelling) and 2.6.2 (groundwater numerical modelling) is conducted for the Clarence-Moreton bioregion.